Contents I. Introduction
As the developments of power electronics, electromagnetic interference (EMI) among electrical equipment becomes serious. So how to cancel the interference and insure the electromagnetic compatibility (EMC) of the electrical equipment have become one aspect that should be considered in the design of the power electronics. EMI filters are very useful for reducing interference. However, most engineers treat the design of EMI filters as “Black Magic”[1] problems, because there are still not universal design methods for EMI filters. The first reason is that a EMI filter does not work in impedance matching conditions. And the measurements of the source and load impedances are difficult because the Z-parameters and Y-parameters are found by either a short circuit or an open circuit. Indeed, the short circuit and the open circuit do not exist in the HF range because of the parasitic parameters. The second reason is the influences of the parasitic parameters. Fig. 1 shows the influence of the parasitic parameters to the insertion loss of a EMI filter. From Fig. 1, the conclusion can be made that the insertion loss (IL) becomes worse because of the parasitic parameters. Besides, the definition of the insertion loss is defined when the source and load impedances are . But in most cases, the is just an ideal number. So, it is confusing to choose EMI filters in these conditions.
